Samanvaya Srivastava publishes on water-like properties of soft nanoparticles
The unusual properties of water, including its anomalous thermal expansion and density anomaly at 4 oC, have intrigued researchers for decades. They are notoriously hard to investigate experimentally owing to the inherently small length scales and complex interactions that appear to govern the phenomena. Studies of colloidal suspension as models for atomic and molecular liquids have shown that some of these anomalies can be engineered in soft colloidal systems. The report, published in the April 5th, 2013 issue of Physical Review Letters, describes anomalous trends in structure and motion of dense suspensions of soft nanoparticles that mirror the anomalies observed in complex liquids like water. The discovery allows for an extension of the toolbox of the experimental physicist interested in employing suspensions for studying interactions and dynamics in molecular liquids.
The team, with members from Cornell University and Argonne National Laboratory, synthesized soft nanocolloids by densely tethering small polymeric ligands onto the surface of silica nanoparticles. Small angle X-ray scattering (SAXS) and X-ray photon correlation spectroscopy (XPCS) measurements were carried out to reveal the equilibrium structure and the characteristic of particle motion at X-ray Science Division Beamline 12-ID-B and Beamline 8-ID-I at the U.S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratories, respectively. It was found that correlation among particles decreases and particle speed up as volume fraction rises above a critical particle loading, coinciding with sharp increase in resistance by these materials to any physical deformation. This is in contrast to the usual situation where increasing the concentration of particles in a dilute suspension decreases the space available for placing new particles, thus increasing the correlation between the particles and slowing them down. “It becomes easier for any particle in these suspensions to diffuse when they are more surrounding by their neighbors; the counterintuitive nature of this situation can be illustrated with the following analogy - it is easier to make a run and score a touch down when the opposing team has fifteen people in defense.”
Such anomalous behavior in water and other complex liquids have been long argued to exist for systems with soft repulsion. The recent report is the first instance of experimental observation of such behavior in a colloidal system. Our empirical findings lend support to an emerging consensus from simulation studies that in systems with soft repulsive interactions, the energy required to compress the particle is offset by the extent of entropic gains, producing the observed set of anomalies.
Samanvaya Srivastava1, Lynden A. Archer1 and Suresh Narayanan2, “Structure and Transport Anomalies in Soft Colloids”, Phys. Rev. Lett. 110, 148302 (2013). DOI: 10.1103/PhysRevLett.110.148302
Author Affiliations: 1School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, USA. 2Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA